In mobile radio systems, the signal propagation occurs in the form of multiway paths whose influence on the signal can be described in a form of a linear transformation which varies over time. Such signal distortions make correct detection of the data transferred between the base station and the mobile radio impossible. For this reason, the channel distortions in the case of data transfer based on the UMTS (Universal Mobile Telecommunications System) standard, for example, are estimated using a pilot signal (Common Pilot Channel; CPICH). The pilot signal is a signal which is transmitted by the base station and is used for continuously transmitting the same pilot symbol or a continuously recurring pattern comprising two different pilot symbols.
In one simple channel model, the symbols rk received by the mobile radio can be described mathematically in the following manner:rk=sk·ck+nk  (1)
In equation (1), sk represents the symbols transmitted from the base station, ck represents the channel parameter and nk represents a noise component. The channel parameter ck describes the rotation-stretching for the symbols sk in the transmission channel. The integer index k indicates the chronological order of the symbols. All variables in equation (1) represent complex numbers.
Equation (1) also applies to the pilot symbols transmitted. If the noise component nk is ignored, then it is possible to ascertain the channel parameter ck by multiplying the received pilot symbols rk by the complex-conjugate known pilot symbols sk. The channel parameter ck obtained in this manner can be used to eliminate the influence of the transmission channel on the transmitted symbols after they have been received in the mobile radio in line with equation (1). However, physical effects (in the radio frequency receiver, inter alia) mean that the signals received are subject to noise, meaning that the channel parameter ck can be estimated only with finite accuracy.
The German patent application with the file reference 102 50 361.3-35 allocated by the German Patent and Trade Marks Office proposes a cascaded and adaptive structure of low-pass filter units which is used to filter the pilot symbols received by the mobile radio. This allows a considerable improvement in the accuracy of the channel estimation. The aforementioned patent application is incorporated hereby in the disclosed content of the present patent application.